As with the Serializable isolation level, the Snapshot level prevents dirty reads, nonrepeatable reads and phantom reads. In addition, other transactions writing data do not block the current transaction for reading data. Snapshot transactions do not request locks when reading data, nor do they block other transactions from writing data. The current transaction works with a snapshot of the data as it existed at the beginning of that transaction. If another transaction modifies data after the start of the current transaction, the data is not visible to the current transaction. SNAPSHOT: A statement can use data only if it will be in a consistent state throughout the transaction.However, it can have the biggest impact on performance, compared to the other isolation levels. As a result, the Serializable isolation level prevents dirty reads, nonrepeatable reads, and phantom reads. No other transaction can modify data being read by the current transaction until it completes, and no other transaction can insert new rows that would match the search condition in the current transaction until it completes. SERIALIZABLE: A query in the current transaction cannot read data modified by another transaction that has not yet committed.However, if another transaction inserts new rows that match the search condition in the current transaction, in between the current transaction accessing the same data twice, phantom rows can appear in the second read. In addition, no other transactions can modify data being read by the current transaction until it completes, eliminating nonrepeatable reads. REPEATABLE READ: A query in the current transaction cannot read data modified by another transaction that has not yet committed, thus preventing dirty reads.Read Committed is the default isolation level for all SQL Server databases. The isolation level uses shared locking or row versioning to prevent dirty reads, depending on whether the READ_COMMITTED_SNAPSHOT database option is enabled. However, data can still be modified by other transactions between issuing statements within the current transaction, so nonrepeatable reads and phantom reads are still possible. READ COMMITTED: A query in the current transaction cannot read data modified by another transaction that has not yet committed, thus preventing dirty reads.It’s also possible for data to be modified by another transaction between issuing statements within the current transaction, resulting in nonrepeatable reads or phantom reads. As a result, it’s possible that a statement will read rows that have been inserted, updated or deleted, but never committed to the database, a condition known as dirty reads. The database engine does not issue shared locks when Read Uncommitted is specified, making this the least restrictive of the isolation levels. READ UNCOMMITTED: A query in the current transaction can read data modified within another transaction but not yet committed.Within the SET TRANSACTION ISOLATION LEVEL statement, you must specify one of the following five isolation levels: The isolation level will apply to the rest of that session, unless you explicitly change the level again. To set the isolation level, you can issue a SET TRANSACTION ISOLATION LEVEL statement after you connect to SQL Server. “How do I set the transaction isolation level when connecting to a SQL Server database?” “Why would I ever use the Read Uncommitted isolation level?”.“What’s the difference between using the NOLOCK table hint and the Read Uncommitted transaction level?”.“How do I verify the transaction isolation levels that an application is using when connecting to the database?”.“How do I verify which snapshot-related database options are enabled on my database?”.“The Serializable and Snapshot isolation levels appear to achieve the same results.“How do I enable the snapshot isolation level on my transactions?”.“How do I set the default transaction isolation level for write operations?”.“What transaction isolation levels can I specify on a memory-optimized table?”.“How do I set the default transaction isolation level on the current database?”.“What is the Read Committed Snapshot isolation level and how do I enable it?”.What do they have to do with concurrency problems?” “I still don’t get all this dirty reads and phantom reads business.“How do I set the transaction isolation level when connecting to a SQL Server database?”.
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